Wow, I have to completely disagree with the first answer simply because the question can be taken in a totally different way! Yes, it could mean the narrator's "first response" to Usher when Madeline appears, ... but that is NOT the narrator's first response! The first response (NOT in person) is through letter. The first response (IN person) is when the narrator arrives at Usher's home. So, let's look at these possibilities, just in case that is what you mean by your question.
Well, the best way to look at this question is to find the quotation and put it in context. The first instance would be when the narrator receives the letter:
A letter, however, had lately reached me in a distant part of the country--a letter from him--which, in its wildly importunate nature, had admitted no other than a personal reply. The writer spoke of acute bodily illness--of a mental disorder, ... which allowed me no room for hesitation; and I accordingly obeyed forthwith.
If this is the "first response" you mean, then I would say the narrator responds with neither anger NOR disregard, but with grave concern. How would he NOT? There is sickness and mental illness mentioned! ... of a dear friend! As the quote implies, the narrator rushes to Usher's side. How loyal of him!
However, you might also mean the "first response" when the narrator first SEES Usher after a long time:
Upon my entrance, Usher arose from a sofa on which he had been lying at full length, and greeted me with a vivacious warmth ... [and] sincerity. We sat down; and for some moments, while he spoke not, I gazed upon him with a feeling half of pity, half of awe. Surely, man had never before so terribly altered, in so brief a period, as had Roderick Usher!
Again, if this is the "first response" you mean, I would have to say is neither anger nor disregard again. The narrator says plainly: "I gazed upon him with a feeling half of pity, half of awe." There you go. There is concern there. No anger. No disregard. He was "wan" and "cadaverous" with eyes "large and liquid." Lips were "thin and very pallid." This goes on and on to achieve Poe's "single effect" of horror. There is still grave concern.
My guess, then, is that you don't mean the narrator's "first response" at all, but the response to Usher's call later in the story. In this case, the first answer is correct.
The narrator treats Roderick's panic with something like disregard. He knows his friend is disturbed and does not believe there is anything to worry about.
When Roderick insists to the narrator that there is something outside, the narrator assumes that Roderick only saw some sort of electrical activity resulting from the weather.
He tries to divert his attention and agitated state with some readings. The narrator reads to him despite Roderick's fidgeting and obvious distress. The narrator hears noises during his reading, but does not give them much thought.
The narrator is finally aware of something wrong when Roderick stands up and shouts that Madeline is there. Madeline is definitely there, in her bloody shroud, and falls on her brother.
The narrator runs from the house as it literally disintegrates.